The Ets family of transcription factors have been recognized as a group of protooncogenes which appear to function in multiple aspects of development and cellular differentiation in a number of organ systems. In particular, these transcription factors have been shown to be important regulators of hematopoietic cell differentiation and function, as well as playing a role in angiogenesis. Our work has focused on understanding the roles of ets-1 and ets-2 during mouse development. We have established the expression patterns for these genes throughout mouse development and have documented a wider role for ets function than previously appreciated. Using transgenic technologies, we have demonstrated that a regulatory region is contained within the first intron of the ets-1 gene which appears to direct transcription to the developing vasculature. Our studies using homologous recombination in embryonic stem cells have demonstrated that the loss of only one allele of ets-1 is lethal during mouse development for unknown reasons. This result is similar to that of experiments in which the VCAM gene has been disrupted through homologous recombination where the loss of only one allele has been shown to be lethal. These projects have been terminated in order to focus upon other high-priority research areas.